Re: preempt-rt, NUMA and strange latency traces

Sébastien Dugué
2006-02-10T13:04:43Z

On Thu, 2006-02-09 at 10:54 -0800, john stultz wrote:
> >
> > I think it's an RSA1 card, but no certainty here. I don't think
> > the x440 came with the RSA2. Is there a way to check for sure without
> > unplugging everything (It's such a mess of cables behind the box)?
>
> No, you're right, the x445 was the box that could have either RSA1 or
> RSA2. The x440 can only have an RSA1.
>
>
> > Whats the SMI issue with the RSA? Could the RSA generate bursts of SMI
> > that could be enough to freeze the CPUs?
>
> SMIs effectively freeze all cpus while the BIOS code executes. With the
> RSA1, there are two main causes of SMIs:
> 1) Periodic SMI: Which occurs every 15 minutes as the RSA checks various
> hardware status and lasts for ~30ms.
>
> 2) Console Redirection SMIs: This occurs only when you use the console
> redirection feature from the RSA. You'll see ~30ms stalls ~once a second
> while the SMI screen-scrapes the console text buffer and sends it out
> over the serial line.

That must be what I've been hitting in my tests and will check soon.

>
> The console redirection one is easy to avoid: just don't use that
> feature if you care about low latencies. The periodic SMI is more
> difficult to work around. With RSA2 based systems, there is a BIOS
> update that allows you to disable this functionality (and on newer
> systems it ships disabled), however I don't believe there is any such
> feature for the older RSA1 based systems.

I'll have to live with it then.

>
>
> > When I ran those tests I was logged on the RSA (serial line). I will
> > try to run the tests again without being connected when I can manage to
> > get some CPU time (eh! shared machine)...
>
> Yes, not using the console redirection feature will definitly help the
> situation, but there still will be the possibility of ~30ms stalls every
> 15 minutes on the x440.

That's the problem and I'll have to synchronize for a proper window
for running my tests.

Is this SMI thing IBM's eyes only stuff or is it documented somewhere?

Thanks for that valuable info.

Sébastien.